Field of the Invention
The present invention is in the field of solid state materials joining. In particular the invention is in the field of joining, coating, or cladding of metals, such as two dissimilar or incompatible materials that are not typically recommended for conventional coating or cladding or joining.
Description of Related Art
Friction-stir processing provides for the solid state joining of pieces of metal at a joint region through the generation of frictional heat at the joint and opposed portions of the metal pieces by cyclical movements of a tool piece that is harder than the metal pieces. An example of this is provided by International Application Publication No. PCT/GB1992/002203. Frictional heat produced between the substrate and the tool during the process causes the opposed portions of the substrate to soften, and mechanical intermixing and pressure cause the two materials to join. Typically, two materials are placed side-by-side and are joined together at the seam between the two.
Additive friction-stir fabrication, invented by the present inventors (see U.S. Pat. Nos. 8,636,194; 8,632,850; 8,875,976; and 8,397,974, the contents of which are hereby incorporated by reference in their entireties), is an additive process for joining materials. Additive friction-stir fabrication (FSF) processes use shear-induced interfacial heating and plastic deformation to deposit metallic coatings onto metal substrates. FSF coatings have bond strengths superior to those of thermally sprayed coatings, and have the potential to enhance corrosion resistance, enhance wear resistance, repair damaged or worn surfaces, and act as an interfacial layer for bonding metal matrix composites. In this process, the coating material, such as a metal alloy, is fed through a rotating spindle or tool to the substrate surface. As shown in FIG. 1, frictional heating occurs at the filler/substrate interface due to the rotational motion of the filler material, such as a rod 23, at an angular velocity ω and the downward pressure P applied. The mechanical shearing that occurs at the interface acts to disperse any oxides or boundary layers, resulting in a metallurgical bond between the substrate 10 and filler material/coating 25. As the substrate 10 moves relative to the tool 15, the coating is extruded under the rotating shoulder of the stirring tool 15.
Conventional techniques are incapable of bonding dissimilar materials such as steel and aluminum. Currently, explosive bonding is used to achieve solid state cladding of dissimilar materials. Explosive bonding uses controlled detonations to accelerate one metal plate into another creating an atomic bond. This method has numerous limitations related to material pairing, thickness and the environment in which it is conducted. Additional techniques include those described by Geiger M. et al, Friction Stir Knead Welding of steel aluminum butt joints, International Journal of Machine Tools & Manufacture 48 (2008) 515-521), and those described in U.S. Patent Application Publication No. 20120273113, U.S. Pat. No. 4,023,613, and Chinese Patent No. CN 102120287 B. Despite these efforts, however, there remains a need in the art for new techniques for joining dissimilar materials.